The present invention relates to a process for producing a carbon fiber mat and an apparatus for carrying out the process, and more specifically relates to a process having excellent production and energy efficiencies, comprising successively melt-spinning a fiber-forming pitch by a centrifugal spinning machine having a horizontal axis of rotation (parallel to the plane on which the spinning machine is placed), forming a mat by the thus spun pitch fibers, bringing the mat into an infusibilized state in an air atmosphere containing NO.sub.2 and calcining the thus infusibilized mat in an inert atmosphere, and an apparatus for carrying out the above-mentioned process.
Hitherto, as a method for melt-spinning a pitch in the production of carbon fibers of pitch series, fixed spinning and centrifugal spinning methods are known, and by such methods, spinning can ordinarily be carried out at a velocity of greater than a few hundred meters per minute, particularly at a velocity of as high as 2000 m/min in the case of the centrifugal spinning method.
However, since in the after-treatment steps including infusibilization, particularly in the step of infusibilization, the reaction velocity is slow and the thus spun pitch fiber is extremely fragile and easily snapped by even a slight impact, the following various methods have been used:
(1) after towing the spun fibers, the towed fibers are hung on a bar on the upper side of a U-shaped tray and treated at every tray,
(2) the spun fibers are piled-up on a mesh belt, or
(3) the spun fibers are once wound on a bobbin, and thereafter, unwound, thereby continuously treated as filaments.
However, in the towing method, particularly in the case of collecting in spun fibers as a tow by using a centrifugal spinning machine having the rotating axis perpendicular to the horizontal plane on which the machine is placed (refer to U.S. Pat. No. 3,776,669, for instance), it is difficult to control the formation of the two. In addition, since in the steps of infusibilization and calcination, the ratio of the volume of the product to the volume of the apparatus must be too small, the efficiency of production is low and the energy consumption is large. The towing method has other demerits that the long term operation of the apparatus is hindered by dirts due to tar and dust and it is difficult to close-up the apparatus.
In the mesh belt method, the production efficiency is low, because the fibers are only piled up, and in the case of increasing the weight/area of the piled fibers by forced ventilation, local damage to the fibers is apt to be caused. In addition, the disadvantages of frequent mechanical troubles relating to the high-speeded traverse and of the difficulty in obtaining stabilized quality have been pointed out.
Besides, in the continuous treatment of the filaments, since it takes a long time to handle the filaments so as to prevent snapping of the filaments, production efficiency is also poor and accordingly, actual practice of such a method is difficult.
As has been stated, every one of the hitherto-proposed methods has a poor production efficiency, and as a result, it is inevitable that the carbon fibers are obtained at the expense of high production costs.
As a result of the present inventors' studies for improving the disadvantages of the conventional processes for producing carbon fibers, it has been found that in the case of forming a pitch fiber mat by melt-spinning a pitch in a centrifugal spinning machine having an axis of rotation provided to be horizontal and piling the thus spun pitch fibers on a belt of a horizontal belt-conveyer which traverses in a parallel direction to the above-mentioned axis of rotation and also moves in the orthogonal direction thereto, a pitch fiber mat in which the fibers have been oriented and which has a sufficient strength is available and the steps of forming the carbon fiber mat including spinning, infusibilizing and calcining are continuously carried out whereby the efficiencies or production and energy are remarkably improved. The present invention has been attained based on the above-mentioned findings.
Accordingly, the objective of the present invention is to provide a process for preparing the carbon fibers derived from pitch, which does not have the disadvantages as in the conventional processes for producing carbon fibers and is high in production and energy efficiencies, namely an economical process which is high in furnace efficiency (kg/m.sup.3 .multidot.hour) represented by the weight (kg) of treated fibers per hour per volume (m.sup.3) of the furnace and low in energy consumption per unit amount of production and which gives a carbon fiber mat directly used as the heat-insulating material or easily processable into chops, milled articles and the like.